1. Field of the Invention
This invention relates to a light measurement device provided with a photoelectric conversion circuit for converting the intensity of incident light from an object to be measured.
2. Description of the Prior Art
Increasingly higher accuracy has been required in recent years for such a light measurement device for use in measurement of white balance of computer displays or TV screens, for example. A photosensor and a photoelectric conversion circuit including an amplifier for amplifying the output of the photosensor which together constitute a light measurement device are susceptible to the effect of temperature variations. Various measures are taken to overcome this problem to achieve a required accuracy.
FIG. 12 shows a typical arrangement used in conventional photoelectric conversion circuits. As shown in the figure, a photosensor PC2 is connected across the inverting and non-inverting inputs of an operational amplifier OP3. The non-inverting input is grounded while the inverting output is connected to an output terminal via a resistor Rf'.
Characteristic lines in FIG. 13 represent relationships between incident light intensity and output voltage VOUT of the photosensor PC2 of the photoelectric conversion circuit shown in FIG. 12 at different temperatures. It is assumed here that the photosensor PC2 has a negative temperature coefficient, that is, the output voltage of the photosensor PC2 decreases as the temperature rises. The output voltage offset level and slope of the characteristic line of the photosensor PC2 vary with change in temperature as shown by L1, L2 and L3 of FIG. 13.
Conventional devices are provided with a thermally sensitive device such as a thermistor in the circuit for temperature compensation. FIG. 14 shows an example of such a circuit.
Referring to FIG. 14, a photosensor PC1 is connected across two input terminals of an operational amplifier OP. The output VOUT of the operational amplifier OP is connected to both an analog-digital converter 20 (hereinafter referred to as ADC) in the succeeding stage and a series circuit including a resistor R and a thermistor RTH. Further, the junction of the resistor R and thermistor RTH is connected to the inverting input of the operational amplifier OP via a resistor RF. The output VOUT is converted into digital data by ADC 20 and delivered to a central processing unit 30 (hereinafter referred to as CPU) which then performs temperature compensation, and is displayed on an indicator 40. Indicated at 50 is a cap which is used to cover the photosensor PC1.
Voltage VRF and output voltage VOUT of the circuit shown in FIG. 14 are given by the following equations, respectively: EQU VRF=-(IP+IB).times.RF+VOFF (1) ##EQU1## where IP is a photoelectric current, IB is a current drawn from the inverting input of the operational amplifier OP, and VOFF is an offset voltage of the operational amplifier OP.
Conventional temperature compensation will be described below.